When is a good age to start?
Because harp technique is a little more complicated than some instruments initially (e.g. piano, recorder), I generally recommend a minimum starting age of around 6-8 years old, depending on how co-ordinated a child is and what kind of attention span they have. This of course is entirely dependent on the individual child. An exceptional child (with a small enough harp to suit them) could start as young as 4, while another child may be better off waiting until they are 10. I often suggest general music programs ("Kinder Music", "Music for Young Children") for children younger than 6, where they can have fun learning about rhythm, tune, and movement without having to worry about a specific technique. At the same time, if there are instruments around the house (or at school, a friend's house, etc.), there's no reason not to let a child explore them regardless of age.
Note on harp size: Keep in mind, that for a small child a small harp is usually best. Even if you have visions of them playing a great big golden-pillared pedal harp some day, a small harp will fit them much better when they are young. A larp harp of between 19-25 strings should be plenty. It will also be less intimidating, cheaper, more portable, and easier to store than a bigger (grown-up sized) harp. Naturally, if you already have a harp at home, there is no harm in letting them try it, under proper supervision. However, you may find that they would also delight in having a harp that is "their size", that they can carry themselves and is just for them.

Are harp lessons right for my child (me)?
Since children learn so quickly, there really is no limit to what type of instrument they can learn on, unless there are unusual physical or other restrictions involved. Some instruments, like the violin, require a bit more control than others; similarly, some woodwind instruments require a certain amount of strength and breath control that very young children may not be capable of yet. This is why young children often start off on instruments like the piano or recorder. However, as with which age to start at, this will largely depend on the individual child.
If your child has expressed interest in the harp specifically, this may be a telling factor. It's good to go with the instrument a child expresses the most interest in, since few things can turn a kid off music more than unwanted lessons in an instrument they don't like (this applies to adults too!). Keep in mind they may change their mind a few times, but if cost is not a problem, then it's good to let them try at least a couple of options. (Of course, you also want to encourage stick-to-it-iveness, and the development of good practice habits - give each instrument a few months at least, before giving up, unless your child really seems to be miserable.)
The main hurdle most people find with the harp, is getting a harp to practice on. Renting is usually the best option, until you are sure that your child wants to pursue the instrument. In my experience kids just love the harp, however only time will tell if that is the main instrument they want to focus on. (Click here for a note on harp size.) If you don't already have a harp to practice on, or one you can borrow, check with your local music stores, music schools, and teachers for rental options. Some harp companies have rent-to-own programs, while others may have "harp financing". There are also inexpensive harp options.
For more ideas on what types of harps to look into, check out our section on "What type of harp should I get?" (it lists different sizes, styles, and prices). For listings of rental harps and used harps, click here. If you're sure you're ready to take the plunge and buy a new harp, check out our "Buying a Harp" section.

Do I have to have previous musical experience, or know another instrument first?
and What instruments are compatible with learning the harp?
You do not have to have any previous music experience to start learning the harp. However, there may be other reasons for starting on another instrument.
If you would like to hold off on harp lessons until your child is older (depending on their current age), then the piano is an excellent instrument to start on. I find it is perfect for teaching young musicians all about basic theory, as it is a very visual as well as tactile instrument. It will
even teach you to read in the same clefs that are required for reading harp music (one of which is the treble cleff, which is the same as for recorder). If you or your child already plays an instrument, chances are you have already picked up some general theory, which will help when learning the harp. If you, for instance, play flute, recorder, or other tune instrument, you probably already know how to read in the treble clef - that means you're halfway there! Similarly, if you play a bass instrument (cello, tuba), you will probably already know the bass clef. Knowing something about note values, keys, and other basic music theory will help you in learning the harp, but this is not necessary. You can approach the harp as a complete musical beginner, just as you could any other instrument. Most beginner harp method books start off from the assumption that you are delving into the music learning process for the first time, and will introduce you to basic theory along with harp technique.
Already having an instrument in the house is of course an added bonus. If you have a piano, you can pick out harp tunes on it, and vise versa. It can also help you while you're learning to tune your harp. If you are having trouble finding a harp, or are waiting for your child to be old enough, consider if you have any other instruments already in your home. Lessons in any instrument will teach a child about music theory, learning the discipline of regular practice, and just generally let them explore music and have fun. I find that music is a cumulative thing - the more angles you explore it from, the more it frees you up to explore new angles, and the easier it is to learn something new.